I rode from our motel just outside the park entrance to the Grand Canyon Village and several points on the rim. From the hotel to the rim was a real slog and I was wondering why I was so slow. The way back to the hotel was REAL fast so I had been going up a slight grade all the way in without it being obvious from the landscape. The altitude does have a noticable effect on us low-landers too so be ready to go a lot slower than you normally expect.
BTW, the entire area is staggeringly beautiful. The Grand Canyon is impressive beyond belief and no amount of seeing photos or films of it can prepare you for the real thing. I envy you.